The invention proceeds from electrodes coated with catalysts and electrode coatings for electrolysis processes for the preparation of chlorine.
The invention relates to novel catalysts, electrode coatings and electrodes for the preparation of chlorine.
Chlorine is usually prepared industrially by electrolysis of sodium chloride or hydrochloric acid or by gas-phase oxidation of hydrogen chloride (Schmittinger, Chorine, Wiley-VCH 1999, pages 19-27). If electrolysis processes are used, the chlorine is produced at the anode. Titanium on the surface of which an electrochemically active catalyst is present is usually used as electrode material for the anode. The catalyst-containing layer on the surface is usually referred to as a coating. The function of the catalyst is to reduce overvoltages and to avoid evolution of oxygen at the anode (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, pages 469-470).
Graphite anodes are used in the preparation of chlorine by electrolysis of hydrochloric acid (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, page 514). In the electrolysis of hydrochloric acid in which, for example, a gas diffusion electrode is used on the cathode side, it is possible to use titanium anodes which have noble metal-based catalysts in the coating (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, page 515).
Electrodes for electrolysis processes are usually based on a metal which belongs to the valve metals. Valve metals are, for example, the metals titanium, zirconium, tungsten, tantalum and niobium which owing to oxide layers on the metal surface act as diode material for electric current.
An electrocatalytically active catalyst comprising a noble metal and/or a metal oxide thereof is usually applied to the surface of the valve metals, with oxides of the valve metal also being able, if appropriate, to be additionally present in the metal oxide (WO 200602843 (ELTECH), BECK, Electrochimica Acta, Vol. 34, No. 6, pages 811-822, 1989)). The oxide-forming noble metal usually belongs to the platinum metals such as iridium, ruthenium, rhodium, palladium, platinum or mixtures thereof. Such electrodes are usually referred to as DSA electrodes (DSA=“dimensionally stable anode”).
Disadvantages of these known electrodes when used in halide-containing electrolytes are the still high overvoltage for chlorine formation, the tendency of the electrodes to nevertheless evolve oxygen, the high electrolysis potential and the need for large amounts of costly noble metal for producing the coating. All these factors have an adverse effect on the economics of the known electrolysis process.
For the coatings of the prior art (DE 602005002661 T2), it is known that the noble metals are eluted from the coating over time under electrolysis conditions, i.e. they are not sufficiently corrosion resistant. The necessity of corrosion resistance is made clear by the fact that the loss of the noble metal-containing coating leads to the electrode metal, usually the valve metal, coming into direct contact with the electrolyte and forming an oxide which does not conduct current on its surface. In an ongoing electrolysis process, this means that electrochemical processes no longer take place on this surface, which can result in total failure with the corresponding economic consequences.
Furthermore, when the electrolyser having noble metal-containing DSA electrodes is used in chloride-containing solutions for the preparation of chlorine, it is observed that the secondary reaction of oxygen formation cannot be fully suppressed, as a result of which oxygen is present in the chlorine. The proportion of oxygen means an increased outlay for purification of the chlorine and therefore likewise has adverse effects on the economics of the electrolysis. The increased formation of oxygen is particularly clearly apparent when the sodium chloride concentration in the electrolyte decreases, in particular at a concentration below 200 g/l of NaCl.
Furthermore, the sole use of noble metals as catalytic material likewise has an adverse effect on the economics of known electrodes because of the high price and decreasing availability on the world market of these metals.
Attempts have been made to use diamond coatings for electrodes in electrochemical processes, e.g. such coatings can be applied to electrodes by CVD processes (chemical vapour deposition). In the case of electrolysis in sodium sulphate anolytes acidified with sulphuric acid, the coating is not stable and flakes off. Furthermore, the coating had defects so that the electrode metal was exposed to electrochemical corrosive attack. (AiF research project 85 ZN, 2003 to 2005, final report for the period 1.01.2003 to 31.03.2005 “Entwicklung and Qualitätssicherung stabiler Diamant-beschichteter Elektroden für neuartige elektrochemische Prozesse”). The research project was stopped because the technical objective was not achieved. (http://www.ist.fraunhofer.de/kompetenz/funktion/diaelektro/Abschlussbericht%20KombiAB2-eingereicht.pdf). We have carried out our own unpublished experiments on the use of electrode coatings containing only diamond in electrolysis. No chlorine has hitherto been able to be evolved from a sodium chloride solution at these diamond structures. Furthermore, it was observed that the diamond layer becomes detached from the metal support under electrolysis conditions.
It was therefore an object of the invention to find a catalyst which enables the electrolysis to be carried out at a lower electrolysis voltage and a lower sodium chloride concentration, with the oxygen content in the chlorine being minimized and the use of noble metal being reduced. A further object was to find a coating which adheres firmly to the metal of the electrode and is not attacked either chemically or electrochemically. A chemically resistant, inexpensive catalyst which has a low noble metal content should likewise be made available for the gas-phase oxidation of hydrogen chloride.